A photodetector comprises a photodetection element or a plurality of photodetection elements. The electric charge output from each photodetection element is integrated by an integration circuit, so that the integration result is output as a voltage. In a certain photodetector, the analog value of this voltage is converted into a digital value (A/D conversion), so that the digital value is output. When the voltage exceeds a predetermined value in this A/D conversion, the digital value output as a result of the A/D conversion of the voltage is saturated at a value corresponding to the predetermined value. This causes a problem where accurate photodetection is not achieved. Thus, in the prior art, the above-mentioned predetermined value is set to be an expected maximum value or larger in order to prevent the saturation.
On the other hand, it is desired to expand the range of the analog value (that is, dynamic range) that can be A/D-converted without saturation. Several techniques for this have been proposed. For example, a technique is known where the dynamic range is expanded using logarithmic compression. In this logarithmic compression technique, a photodiode (photodetection element) and a MOS transistor are connected. Then, when the electric potential of the connection point between these two components is denoted by “V,” and when the electric current flowing from the photodiode to the MOS transistor is denoted by “I,” the relational expression I=A·exp(q(V−Vth)/kT) holds and is used. Here, “A” indicates a proportional constant; “q” indicates the amount of electric charge of the electron; “Vth” indicates the threshold voltage of the MOS transistor; “k” indicates Boltzmann's constant; and “T” indicates the absolute temperature. As seen from this relational expression, for a low incident light intensity (that is, for a small I), a large change is caused in the output voltage V by a change in the incident light intensity. On the contrary, for a high incident light intensity (that is, for a large I), a small change is caused in the output voltage V by a change in the incident light intensity. By virtue of this, the dynamic range is expanded.